xtag-sp-debug-reason: Processing queue temporary exceeded for this host

If you've encountered this response header while checking your site, it means our servers have temporarily throttled the optimization queue.

Here at ShortPixel, we have an internal queue throttling mechanism per domain. If a new and high traffic website is using our CDN services and has a lot of files to optimize, the servers could become overloaded, which could also affect the optimization speed for other customers if we don't implement this. The queuing mechanism works similar to the following:

  • 1 JPG = 1 point
  • 1 PNG or WebP = 5 points
  • 1 AVIF = 10 points

In other words, processing a PNG or WebP is 5 times more expensive than processing a normal JPG and an AVIF is 10 times more expensive. To make this clear: This is not something you can improve as a customer; these are technical limitations because the servers need much more time to process a PNG/WebP/AVIF than a JPG.

Also keep in mind that this only happens when an image is requested for processing. Each subsequent visit is not affected by this mechanism, even if you have hundreds or thousands of visits per hour; on these visits, the already processed image is only served by the CDN and not requested for optimization.

As for the queue limit: if you need to process 100 JPG files (or 10 AVIF files for comparison) at the same time for the same domain, the header will appear and the optimization queue will be temporarily paused for 5-10 seconds until some of the files in the queue are processed and new ones can be added to the queue for the same user. All this happens fully automatically and is usually not noticeable by the end user.

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